The present invention relates to the field of communications and more particularly to receiving radio communications.
A radio channel for a mobile terminal in a cellular radiotelephone communications system may be difficult to operate. In particular, the transmitted signals are often reflected, scattered, diffracted, delayed, and attenuated by the surrounding environment. Moreover, the radio channel for a mobile terminal is often not stationary because of movement of the mobile terminal and movement of objects near the mobile terminal. The mobile terminal may move rapidly when used in an automobile, and other vehicles may also be in motion near the mobile terminal.
Characteristics of the radio channel may also vary from one area to another due to differences in terrain/buildings, climate, and/or other factors. The propagation of a radio signal along the radio channel may thus be subject to multi-path fading, shadowing, and path loss. Of these factors, multi-path fading may be the most significant, and multi-path fading can be characterized by envelope fading, Doppler spread, and time-delay spread.
Doppler shift is the frequency shift experienced by the radio signal when the mobile terminal is in motion, and the Doppler spread is a measure of the spectral broadening caused by the time rate of change of the mobile radio channel. Doppler spread thus leads to frequency dispersion, and the Doppler spread in the frequency domain is closely related to the rate of change in the observed signal. The adaptation time of an algorithm used in an adaptive receiver should thus be faster than the rate of change of the channel to be able to accurately track the fluctuations in the received signal.
A mobile terminal in a DAMPS cellular radiotelephone communications system, for example, may experience a Doppler spread in the range of 0 Hz to 250 Hz depending on the vehicle speed, carrier frequency, and other factors. Knowledge of the rate of change of the radio channel can be used to improve receiver performance and/or reduce receiver complexity. Moreover, the adaptation parameters for an adaptive receiver can be varied as a function of the Doppler spread. Instead of fixing the tracker and interpolation parameters for the worst case expected Doppler spread, for example, the parameters can be changed adaptively as a function of the Doppler information to improve performance. Similarly, Doppler spread information can be used to control the receiver adaptively for different speeds at which the mobile may be traveling. In other words, different receiver algorithms can be used depending on the speed at which the mobile terminal is traveling.
Estimates of the Doppler spread can thus be used to increase receiver performance. The parameters of a receiver adaptation algorithm can be varied as a function of the Doppler spread to adaptively optimize a coherent detector in a receiver, for example. In addition, the hand-off process in a cellular mobile telephone system can be enhanced if an estimate of the Doppler spread is available. Handoff of a fast moving mobile terminal to a micro cell can thus be avoided.
Doppler spread estimation is discussed, for example, in U.S. Pat. No. 4,723,303 to Koch entitled xe2x80x9cMETHOD OF AND CIRCUIT ARRANGEMENT FOR MEASURING THE QUALITY OF RADIO-TRANSMISSION CHANNELS OF A RADIO-TRANSMISSION SYSTEMxe2x80x9d, and U.S. Pat. No. 5,016,017 to Raith entitled xe2x80x9cMETHOD OF CONTROLLING THE FREQUENCY OF A COHERENT RADIO RECEIVER AND APPARATUS FOR CARRYING OUT THE METHODxe2x80x9d. The disclosures of each of these patents is hereby incorporated herein in their entirety by reference.
A method of estimating Doppler spreads from a sequence of channel estimates is discussed, for example, in the thesis by Lars Lindbom entitled xe2x80x9cAdaptive Equalization For Fading Mobile Radio Channelsxe2x80x9d, (Techn.Lic.Thesis No. UPTEC 92124R, November 1992, Department of Technology, Uppsala University, Uppsala Sweden), the disclosure of which is hereby incorporated herein in its entirety by reference. In the Lindbom thesis, differentials of the channel estimate, which comprise differences of values between two points in time, are used to estimate Doppler spreads. These differentials, however, may be noisy so that averaging may be needed. As a result, the averaging may give biased estimates of the Doppler spread.
Other Doppler spread estimation techniques are discussed in the reference by Karim Jamal et al. entitled xe2x80x9cAdaptive MSLE Performance On The D-AMPS 1900 Channelxe2x80x9d (IEEE Trans. Vehic. Technol., vol. 46, August 1997), and the reference by M. Morelli et al. entitled xe2x80x9cFurther Results In Carrier Frequency Estimation For Transmissions Over Flat Fading Channelsxe2x80x9d (IEEE Commun. Letters, vol. 2, pp. 327-330, December 1998). The disclosures of these references are also incorporated herein in their entirety by reference.
Notwithstanding the approaches discussed above, there continues to exist a need in the art for improved Doppler spread estimation approaches.
It is therefore an object of the present invention to provide improved methods of estimating Doppler spreads for communications channels and related systems and receivers.
It is another object of the present invention to provide less complex methods of estimating Doppler spreads and related systems and receivers.
These and other objects can be provided according to the present invention by providing an estimate of the communications channel and generating an autocorrelation function for the estimate of the communications channel. One of a plurality of autocorrelation function hypotheses is selected to approximate the autocorrelation function for the estimate of the communications channel wherein each of the autocorrelation function hypotheses corresponds to a respective Doppler spread estimate hypothesis. The Doppler spread estimate hypothesis corresponding to the selected autocorrelation function hypothesis is then selected as an estimate of the Doppler spread for the communications channel.
The autocorrelation function hypotheses can thus be saved in a memory of a Doppler spread estimator according to the present invention and compared to the autocorrelation function of the estimate of the communications channel, with the closest autocorrelation function hypotheses being accepted as an estimate of the actual autocorrelation function for the communications channel. The Doppler spread hypothesis corresponding to the closest autocorrelation function can thus be used as an estimate of the actual Doppler spread for the communications channel. The complexity of calculations used to estimate the Doppler spread can thus be reduced while providing a relatively accurate estimate of the Doppler spread. In addition, the number of autocorrelation function hypotheses used can be increased to provide a more accurate estimation, or decreased to reduce the number of calculations and amount of memory used.
In greater detail, the selection of one of the autocorrelection function hypotheses can include comparing the autocorrelation function for the estimate of the communications channel with each of the plurality of autocorrelation function hypotheses. In addition, the selection of one of the autocorrelation function hypotheses can include selecting one of the plurality of autocorrelation function hypotheses most closely approximating the autocorrelation function for the estimate of the communications channel.
Moreover, the selection of one of the autocorrelation function hypotheses can include generating a plurality of error signals respectively corresponding to the plurality of autocorrelation function hypotheses wherein each of the error signals represents a difference between the respective autocorrelation function hypothesis and the autocorrelation function of the estimate of the communications channel, and comparing the error signals to select the autocorrelation hypothesis to approximate the autocorrelation function for the estimate of the communications channel. In particular, the error signals can be compared to choose the error signal representing a least difference between the corresponding autocorrelation function hypothesis and the autocorrelation function for the estimate of the communications channel. In addition, the comparison of the error signals can be preceded by averaging each of the error signals to provide averaged error signals wherein comparing the error signals comprises comparing the averaged error signals.
Methods, systems, and receivers according to the present invention can thus be used to provide estimates of Doppler spreads for a communications channel with reduced complexity.